Wafer bonding using reactive foils for massively parallel micro-electromechanical systems packaging

ABSTRACT

A method for forming device packages includes forming a perimeter comprising a reactive foil and a bonding material interposed between a first wafer and a second wafer, pressing the first and the second wafers against the reactive foil and the bonding material, initiating the reactive foil, wherein the reactive foil heating the bonding material to create a bond between the first and the second wafers, and singulating the first and the second wafers into the device packages.

FIELD OF INVENTION

[0001] This invention relates to the use of reactive foils for waferbonding and for forming device packages.

DESCRIPTION OF RELATED ART

[0002] Wafer bonding techniques are used in IC (integrated circuit) andMEMS (micro-electromechanical systems) manufacturing. By achievingpackage function at the wafer level, it is possible to realize costsavings via massive parallel assembly. While MEMS packaging has beenincorporated at the device fabrication stage of the micromachiningprocess, there is a need for a more uniform packaging process to producehigher yields and to lower costs. Hermeticity and low-temperaturesealing are two key elements that present formidable challenges to thegoal of process uniformity.

[0003] MEMS devices and IC's are generally fragile devices that aresensitive to high temperatures and high voltages required forconventional wafer bonding techniques. Conventional wafer bondingtechniques include anodic bonding, intermediate-layer bonding, anddirect bonding. Anodic bonding typically takes place at 300 to 450° C.and requires the application of high voltages. Direct bonding typicallytakes place at 1000° C. and requires extremely good surface flatness andcleanliness. Intermediate-layer bonds are typically formed with brazesor solders such as AuSi (gold silicon), AuGe (gold germanium), and AuSn(gold tin). All of these brazes and solders have melting temperaturesthat can degrade temperature sensitive materials and devices.

[0004] Thus, what is needed is a method that bonds wafers withoutexposing MEMS devices and IC's to high temperatures and high voltages.

SUMMARY

[0005] In one embodiment of the invention, a method for forming devicepackages includes forming a perimeter comprising a reactive foil and abonding material interposed between a first wafer and a second wafer,pressing the first and the second wafers against the reactive foil andthe bonding material, initiating the reactive foil whereby the reactivefoil heats the bonding material to create a bond between the first andthe second wafers, and singulating the first and the second wafers intothe device packages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIGS. 1, 2, and 3 illustrate cross-sections of wafer structuresfor forming device packages in various embodiments of the invention.

[0007]FIG. 1A illustrates a top view of the pattern formed by thereactive foil and the bonding material around devices on a wafer in oneembodiment of the invention.

[0008]FIGS. 4A, 4B, 4C, and 4D illustrate the resulting cross-sectionsof a wafer structure formed by a method in one embodiment of theinvention.

[0009]FIGS. 5A and 5B illustrate the resulting cross-sections of a waferstructure formed by a method in another embodiment of the invention.

[0010]FIG. 6 illustrates the resulting cross-section of a waferstructure formed by a method in another embodiment of the invention.

[0011]FIGS. 7A and 7B illustrate the resulting cross-sections of a waferstructure formed by a method in another embodiment of the invention.

[0012]FIG. 8 illustrates the resulting cross-section of a waferstructure formed by a method in another embodiment of the invention.

[0013]FIGS. 9A and 9B illustrate alternative layer geometries for thereactive foil and the bonding material in embodiments of the invention.

DETAILED DESCRIPTION

[0014] In accordance with one aspect of the invention, devices packagesare formed with reactive foils at both the die and wafer levels. In oneembodiment of the invention, a reactive foil and a bonding material arepatterned to form one or more perimeters around devices atop a firstwafer. A second wafer is aligned with the first wafer and the reactivefoil is initiated to start an exothermic reaction that releases heat.The heat from the reactive foil is localized away from the devices. Theheat melts the bonding material to form a bond between the first and thesecond wafers. The bonded wafer structure is singulated to formindividual packages containing the devices.

[0015]FIG. 1 illustrates a cross-section of a structure 10 that formsone or more device packages in one embodiment of the invention.Structure 10 includes a base wafer 12 and a cover wafer 14. Depending onthe application, one or both of wafers 12 and 14 can have bulk orsurface micro-machined features. Micro-machined features includeelectronic devices, electromechanical sensors, micro-actuators, opticalcomponents, and mechanical alignment marks.

[0016] A reactive foil 16 is formed atop wafer 12. Reactive foil 16includes alternating layers of reactive materials (e.g., aluminum andnickel) that produces an exothermic reaction when initiated. Reactivefoil 16 can be formed atop wafer 12 by magnetron sputtering andpatterned by methods described later in the present disclosure. For moredetails on reactive foils, please refer to (1) International ApplicationNo. PCT/US01/14052, International Publication No. WO 01/83205, publishedNov. 8, 2001, (2) International Application No. PCT/US01/14053,International Publication No. WO 01/83182, (3) U.S. patent applicationSer. No. 09/846,422, US Patent Application Publication No. US2001/0038029, published Nov. 8, 2001, (4) U.S. patent application Ser.No. 09/846,447, US Patent Application Publication No. US 2001/0046597,published Nov. 29, 2001.

[0017] A bonding material 18 is next formed atop reactive foil 16.Bonding material 18 can be a solder, a braze, or any other bonding agentrequiring heat to transform the bonding material into its final state.Bonding material 18 can be formed atop reactive foil 16 by evaporationor sputtering. Reactive foil 16 and bonding material 18 are thenpatterned to form part of one or more perimeters 17 (FIG. 1A) arounddevice areas 19 (FIG. 1A) on wafer 12. Wafers 12 and 14 are then alignedto the required accuracy. A nominal force is applied to press wafers 12and 14 against reactive foil 16 and bonding material 18. This preventsmovement of wafers 12 and 14 when reactive foil 16 is initiated.Reactive foil 16 is initiated to provide a localized heat source forbonding material 18. As a result, bonding material 18 bonds wafers 12and 14. After the exothermic reaction, reactive foil 16 leaves behind anintermetallic mixture consisting of the materials from the reactive foil(e.g., aluminum and nickel) and the bonding material.

[0018] Alternatively, bonding material 18 (e.g., solder) can bedeposited atop reactive foil 16 by plating or screen printing, insteadof evaporation or sputtering, after reactive foil 16 has been patterned.A photoresist, such as a plating or screen printing mask, is firstapplied to areas that do not need solder and then the entire device orwafer is plated or screen printed with bonding material 18 to form thedesired bond lines. The photoresist mask is then cleaned off. Plating orscreen printing offers a cost advantage over evaporation and sputteringof the bonding material.

[0019] Bonding wafers 12 and 14 form a partial package for an electronicdevice such as an IC laser, a MEMS device such as an electromechanicalsensor, or an optoelectronic device such as a semiconductor laser (e.g.,Fabry-Perot, distributed feedback, vertical cavity surface emittinglasers), light emitting diodes, and photodetectors (e.g.,positive-intrinsic-negative photodetectors and monitor diodes). Afterthe reactive foil bonding, a device 20 is placed, aligned, and bonded tobase wafer 12 through a hole 22 in cover wafer 14. Although not shown inFIG. 1, one skilled in the art understands there are multiple devices 20in structure 10. Structure 10 is singulated to form individual devicepackages.

[0020]FIG. 2 illustrates a cross-section of a structure 50 that formsone or more device packages in another embodiment of the invention.Structure 50 includes base wafer 12 and a cover wafer 52. In thisembodiment, base wafer 12 has device 20 constructed thereon or placed,aligned, and bonded prior to reactive foil bonding. Furthermore, coverwafer 52 has a cavity 54 to accommodate device 20. Depending on theapplication, wafers 12 and 14 can also have other bulk or surfacemicro-machined features.

[0021] As similarly described above, wafers 12 and 52 are bonded usingreactive foil 16 and bonding material 18. Bonding wafers 12 and 52 mayform a complete hermetic package for electronic and MEMS devices. As oneskilled in the art understands, structure 50 may include multipledevices 20 and may be singulated to form individual device packages.

[0022]FIG. 3 illustrates a cross-section of a structure 80 that formsone or more device packages in another embodiment of the invention.Structure 80 includes base wafer 12, an intermediate ring wafer 82, anda cover wafer 84. Depending on the application, wafers 12, 82, and 84can have bulk or surface micro-machined features.

[0023] Wafers 12 and 82 are bonded using reactive foil 16 and bondingmaterial 18. Reactive foil 16 and bonding material 18 are formed atopwafer 12 and then patterned to form perimeters 17 (FIG. 1A) on wafer 12.Wafers 12 and 82 are then aligned to the required accuracy. A nominalforce is applied to press wafers 12 and 82 against reactive foil 16 andbonding material 18. This prevents movement of wafers 12 and 82 whenreactive foil 16 is initiated. Reactive foil 16 is then initiated and,as a result, bonding material 18 creates a bond between wafers 12 and82.

[0024] Device 20 is next placed, aligned, and bonded to base wafer 12through a hole 86 in ring wafer 82. Wafers 82 and 84 are then bondedusing a reactive foil 16A and a bonding material 18A. Reactive foil 16Aand bonding material 18A are formed atop cover wafer 84 and thenpatterned to form perimeters on cover wafer 84. These perimeters oncover wafer 84 correspond to and are located opposite of perimeters 17(FIG. 1A) on wafer 12. Wafers 82 and 84 are then aligned to the requiredaccuracy. A nominal force is applied to press wafers 82 and 84 againstreactive foil 16A and bonding material 18A. This prevents movement ofwafers 82 and 84 when reactive foil 16A is initiated. Reactive foil 16Ais then initiated and, as a result, bonding material 18A creates a bondbetween wafers 82 and 84.

[0025] Bonding wafers 12, 82, and 84 may form a complete hermeticpackage for electronic and MEMS devices. As one skilled in the artunderstands, structure 80 may include multiple devices 20 and may besingulated to form individual device packages.

[0026] In accordance with another aspect of the invention, a method isprovided to pattern the reactive foil. Patterning the reactive foil isdifficult because it is reactively thick (e.g., 20 to 100 microns)compared to conventional metal layers (e.g., 1 micron) in semiconductorprocessing. In one embodiment of the invention, two layers ofphotoresist form a lift-off mask used to pattern the reactive foil. Inanother embodiment of the invention, a mechanical lift-off mask is usedto pattern the reactive foil. In yet another embodiment of theinvention, a lithographic etch is used to pattern the reactive foil.

[0027]FIGS. 4A to 4D illustrate a method to pattern a reactive foil inone embodiment of the invention. Referring to FIG. 4A, a firstphotoresist layer 112 is formed atop a wafer 114. A second photoresistlayer 116 is then formed atop of photoresist 112. The material ofphotoresist 116 is selected to develop at a slower rate than photoresist112. Regions 122 of photoresist 112 and regions 124 of photoresist 116are exposed to light 118 through a mask or reticle 120. Photoresists 112and 116 are then developed to form windows 126 through photoresists 116and 112 as shown in FIG. 4B. As a result of the different developmentrate, photoresist 116 forms overhangs 127 above photoresist 112.

[0028] Referring to FIG. 4C, a reactive foil 128 is deposited atopphotoresist 116 and through windows 126 onto wafer 114. Reactive foil128 can be deposited by sputtering. Bonding material 130 is nextdeposited atop reactive foil 128. Bonding material 120 can be depositedby evaporation or sputtering. Overhangs 127 (FIG. 4B) prevents acontinuous layer of reactive foil 128 and bonding material 130 to formover wafer 114.

[0029] Referring to FIG. 4D, photoresists 116 and 112 are stripped, andreactive foil 128 and bonding material 130 thereon are lifted off. Theremaining reactive foil 128 and bonding material 130 on wafer 114 formthe desired bond pattern (e.g., perimeters 17 in FIG. 1A) between wafer114 and another wafer (e.g., a ring or a cover wafer).

[0030] In another embodiment, instead of using two photoresist layerswith different development rates, the top photoresist layer can betreated with chlorobenzene to reduce its development rate to achieve theundercut profile. Alternatively, a single thick photoresist layer canhave its top surface treated with chlorobenzene to achieve the undercutprofile.

[0031] In yet another embodiment, bonding material 130 is plated orscreen printed on top of reactive foil 128 after reactive foil 128 alonehas been deposited and patterned with the steps shown in FIGS. 4A to 4D.

[0032]FIGS. 5A and 5B illustrate another method to pattern a reactivefoil in one embodiment of the invention. Referring to FIG. 5A,photoresist layer 112 is formed atop wafer 114. Photoresist 112 ispatterned by exposing regions 122 to light 118 through a mask or reticle164.

[0033] Referring to FIG. 5B, photoresist layer 116 is formed atopphotoresist 112. In this embodiment, photoresists 112 and 116 can havethe same development rate. Photoresist 116 is patterned by exposingregions 124 to light 118 through a mask or reticle 168. Reticle 168 hassmaller windows than reticle 164. Thus, region 124 is smaller thanregion 122.

[0034] Regions 122 and 124 are then developed to form windows 126 withoverhangs 127 as shown in FIG. 4B. Reactive foil 128 and bondingmaterial 130 are next formed and then patterned with the remainingphotoresists 112 and 116 as described above in reference to FIGS. 4C and4D. Alternatively, bonding material 130 can be plated or screen printedon top of reactive foil 128 after reactive foil 128 alone has beendeposited and patterned with the steps shown in FIGS. 5A and 5B.

[0035]FIG. 6 illustrates another method to pattern a reactive foil inone embodiment of the invention. In this embodiment, a mechanical maskor stencil 192 is used to pattern reactive foil 128 and bonding material130. Mask 192 can be made of stainless steel, glass, or silicon waferinto which undercut windows 194 are machined or etched. Reactive foil128 and bonding material 130 are deposited through windows 194 to formthe desired bond pattern. The excess reactive foil 128 and bondingmaterial deposited on mask 192 can be stripped so mask 192 can bereused. Alternatively, bonding material 130 can be plated or screenprinted on top of reactive foil 128 after reactive foil 128 alone hasbeen deposited and patterned with the steps shown in FIG. 6.

[0036]FIGS. 7A and 7B illustrate another method to pattern a reactivefoil in one embodiment of the invention. Referring to FIG. 7A, reactivefoil 128 is formed over wafer 114. Bonding material 130 is formed atopreactive foil 128. Bonding material 130 can be deposited on top ofreactive foil 128 by evaporation or sputtering. Alternatively bondingmaterial 130 can be plated or screen printed on top of reactive foil128. A photoresist layer 220 is formed atop bonding material 130.Photoresist 220 is then patterned by exposing regions 222 to light 224through a mask or reticle 226.

[0037] Regions 222 are then developed to form etching windows 228 inphotoresist 220. Regions 128A and 130A left unprotected by the remainingphotoresist 220 are etched away, and the remaining photoresist 220 isstripped, to form the structure shown in FIG. 4D.

[0038] In accordance with another aspect of the invention, a reactivefoil is used to bond a large number of devices, such as IC lasers, in aparallel fashion. Referring to FIG. 8, wafer 114 is bulk or surfacemicro-machined according to the specific application. A metal layer 250is formed and patterned to form metal lines atop wafer 114. A reactivefoil 252 and a conductive bonding material 254 are formed and patternedatop metal layer 250. Reactive foil 252 and bonding material 254 can bepatterned by any of the methods described above.

[0039] Devices 256 are aligned and placed atop bonding material 254. Anominal force is applied to press device 256 and wafer 114 againstreactive foil 252 and bonding material 254. This prevents movement ofdevices 256 and wafer 114 when reactive foil 252 is initiated. Reactivefoil 252 is then initiated to heat bonding material 254. As a result,bonding material 254 forms a bond between devices 256 and theircorresponding metal 250. After the exothermic reaction, reactive foil252 leaves behind an intermetallic mixture consisting of the materialsfrom the reactive foil (e.g., aluminum and nickel) and the bondingmaterial. The steps described above can be used to bond the devices inFIGS. 1, 2, and 3 to their base wafers.

[0040] In the embodiments described above, the layer geometry consistsof a bonding material (e.g., solder) on top of a reactive foil. However,different layer geometries can be used to bond the wafers and to formdevice packages. FIG. 9A illustrates a layer geometry where reactivefoil 128 is formed on top of bonding material 130 interposed betweenwafers 114A and 114B to bond the wafers. FIG. 9B illustrates a layergeometry where a sandwich of bonding material 130A, reactive foil 128,and bonding material 130B is interposed between wafers 114A and 114B tobond the wafers. Furthermore, this layer geometry may be repeated.

[0041] Various other adaptations and combinations of features of theembodiments disclosed are within the scope of the invention. Numerousembodiments are encompassed by the following claims.

What is claimed is:
 1. A method for forming device packages, comprising:forming a perimeter comprising a reactive foil and a bonding materialinterposed between a first wafer and a second wafer; pressing the firstand the second wafers against the reactive foil and the bondingmaterial; initiating the reactive foil, the reactive foil heating thebonding material to create a bond between the first and the secondwafers; and singulating the first and the second wafers into devicepackages.
 2. The method of claim 1, wherein said forming a perimetercomprises: forming a first photoresist layer atop the first wafer, thefirst photoresist layer having a first development rate; forming asecond photoresist layer atop the first photoresist layer, the secondphotoresist layer having a second development rate, the seconddevelopment rate being slower than the first development rate; exposingthe first and the second photoresist layers to a pattern; removingexposed regions of the first and the second photoresist layers to form awindow to the first wafer, the window including an undercut profile;depositing the reactive foil atop unexposed regions of the first andsecond photoresist layers and through the window onto the first wafer;and stripping the unexposed regions of first and the second photoresistlayers to lift off the reactive foil thereon.
 3. The method of claim 2,wherein said forming a perimeter further comprises: depositing thebonding material on the reactive foil through the window, wherein saidstripping also lifts off the bonding material on the first and thesecond photoresist layers.
 4. The method of claim 2, wherein saidforming a perimeter further comprises: patterning a mask on the firstwafer; depositing the bonding material over the first wafer, whereinsaid depositing is a step selected from the group consisting of platingand screen printing; and stripping the mask to lift off the bondingmaterial thereon.
 5. The method of claim 1, wherein said forming aperimeter comprises: forming a first photoresist layer atop the firstwafer; defining a first region of the first photoresist layer to beremoved; forming a second photoresist layer atop the first photoresistlayer; defining a second region of the second photoresist layer to beremoved, the second region being above the first region, the secondregion being smaller than the first region; removing the first and thesecond regions to form a window to the first wafer, the window includingan undercut profile; depositing the reactive foil through the windowonto the first wafer; and stripping remaining regions of first and thesecond photoresist layers to lift off the reactive foil thereon.
 6. Themethod of claim 5, wherein said forming a perimeter further comprises:depositing the bonding material on the reactive foil through the window,wherein said stripping also lifts off the bonding material on the firstand the second photoresist layers.
 7. The method of claim 5, whereinsaid forming a perimeter further comprises: patterning a mask on thefirst wafer; depositing the bonding material over the first wafer,wherein said depositing is a step selected from the group consisting ofplating and screen printing; and stripping the mask to lift off thebonding material thereon.
 8. The method of claim 1, wherein said forminga perimeter comprises: placing a mechanical mask over the first wafer,the mechanical mask comprising a window, the window including anundercut profile; depositing the reactive foil through the window ontothe first wafer.
 9. The method of claim 8, wherein said forming aperimeter further comprises: depositing the bonding material on thereactive foil through the window, wherein said stripping also lifts offthe bonding material on the first and the second photoresist layers. 10.The method of claim 8, wherein said forming a perimeter furthercomprises: patterning a mask on the first wafer; depositing the bondingmaterial over the first wafer, wherein said depositing is a stepselected from the group consisting of plating and screen printing; andstripping the mask to lift off the bonding material thereon.
 11. Themethod of claim 1, wherein said forming a perimeter comprises:depositing the reactive foil onto the first wafer; forming a photoresistlayer atop the reactive foil; patterning the photoresist layer to definea window; and removing a region of the reactive foil exposed through thewindow.
 12. The method of claim 1, wherein the second wafer defines ahole, the method further comprising: forming a second reactive foil anda second bonding material interposed between a device and the firstwafer; placing the device through the hole in the second wafer and ontothe first wafer; pressing the device and the first wafer against thesecond reactive foil and the second bonding material; and initiating thesecond reactive foil, the second reactive foil heating the secondbonding material to create a second bond between the device and thefirst wafer.
 13. The method of claim 1, wherein the first wafer includesa device at a first location and the second wafer defines a cavity at asecond location opposite of the first location.
 14. The method of claim13, further comprising: forming a second reactive foil and a secondbonding material interposed between the device and the first wafer;pressing the device and the first wafer against the second reactive foiland the second bonding material; and initiating the second reactivefoil, the second reactive foil heating the second bonding material tocreate a second bond between the device and the first wafer.
 15. Themethod of claim 1, further comprising: forming a second perimetercomprising a second reactive foil and a second bonding materialinterposed between the second wafer and a third wafer; pressing thesecond and the third wafers against the second reactive foil and thesecond bonding material; and initiating the second reactive foil, thesecond reactive foil heating the second bonding material to create asecond bond between the second and the third wafers.
 16. The method ofclaim 15, wherein the first wafer includes a device located at a firstlocation and the second wafer defines a hole located at a secondlocation opposite of the first location.
 17. The method of claim 16,further comprising: forming a third reactive foil and a third bondingmaterial interposed between the device and the first wafer; compressingthe device and the first wafer against the third reactive foil and thethird bonding material; and initiating the third reactive foil, thethird reactive foil heating the third bonding material to create a thirdbond between the device and the first wafer.
 18. The method of claim 1,wherein said forming a perimeter comprises: forming the bonding materialatop the first wafer; and forming the reactive foil atop of the bondingmaterial.
 19. The method of claim 18, wherein the perimeter furthercomprises a second bonding material, said forming a perimeter furthercomprising forming the second bonding material atop of the reactivefoil.
 20. A device package, comprising: a first wafer; a second wafer;and a perimeter of an intermetallic mixture interposed between the firstand the second wafers, the intermetallic mixture comprising materialsfrom a reactive foil and a bonding material, the intermetallic mixturebeing formed after an exothermic reaction of the reactive foil.
 21. Thedevice package of claim 20, further comprising: a device; and a secondintermetallic mixture interposed between the device and the first wafer,the second intermetallic mixture comprising materials from a secondreactive foil and a second bonding material, the second intermetallicmixture being formed after a second exothermic reaction of the secondreactive foil.
 22. The device package of claim 20, wherein the secondwafer comprises a feature selected from the group consisting of a holeand a cavity.
 23. The device package of claim 20, further comprising: athird wafer; and a second perimeter of a second intermetallic mixtureinterposed between the second and the third wafers, the secondintermetallic mixture comprising materials from a second reactive foiland a second bonding material, the intermetallic mixture being formedafter a second exothermic reaction of the second reactive foil.
 24. Amethod for bonding devices to wafers, comprising: patterning a metallayer on a wafer to form metal lines; patterning a reactive foil and abonding material interposed between a device and the metal lines on thewafer; pressing the device and the wafer against the reactive foil andthe bonding material; and initiating the reactive foil, the reactivefoil heating the bonding material to create a bond between the deviceand the metal layer on the wafer.
 25. The method of claim 24, whereinthe device is selected from the group consisting a MEMS(micro-electromechanical systems) device, an electronic device, and anoptoelectronic device.
 26. A device wafer package, comprising: a waferhaving metal lines; a device; and an intermetallic mixture interposedbetween the device and the metal lines, the intermetallic mixturecomprising materials from a reactive foil and a bonding material, theintermetallic mixture being formed after an exothermic reaction of thereactive foil.
 27. The package of claim 26, wherein the device isselected from the group consisting a MEMS (micro-electromechanicalsystems) device, an electronic device, and an optoelectronic device.